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Tempering.                                 125

as prussiate of potash, leather or hoof scraps, but the process is
chemically the same. After packing, which must be carefully
done, to prevent the articles bending while hot, the heat is raised
during two hours, the whole kept at a regular temperature for

about nineteen hours, and then allowed another two hours to
cool, Hernoving the articles they are quenched in water,
straightened, and re-polished. (See Appendices /., //, and F.9
/A 74, ^03, andwz.) ^

Steel of a mild quality may be hardened at the surface by the
absorption of more carbon.

Such small articles as have to withstand considerable wear are
case-hardened, <?,/., radius links for reversing gear.

Tempering is a method of giving to a piece of steel any

required degree of hardness.     Properly there are two distinct

meant when we speak of(tempering' a steel tool-    The